Examination of portions of the eye has been widely practiced for the purpose of early diagnosis of life-style related diseases and various kinds of diseases ranking high in causes of blindness. In such examination on portions of the eye, it is required to find a disease in any portion of the entire eye, and hence it is required to capture an image covering a wide range of portions of the eye (a wide-area fundus image).
In general, a wide-area fundus image is captured by using a fundus camera or SLO (Scanning Laser Ophthalmoscope). Recently, the use of tomography apparatuses such as those used in optical coherence tomography has been proposed (see, for example, A. F. Fercher, C. K. Hitzenberger, G. Kamp, and S. Y. Elzaiat, “Measurement of Intraocular Distances by Backscattering Spectral Interferometry”, Optics Communications 117, 43 (1995) and E. C. W. Lee, J. F. de Boer, M. Mujat, H. Lim, and S. H. Yun, “In vivo optical frequency domain imaging of human retina and choroid”, Optics Express Vol. 14, No. 10 (2006). Note that optical coherence tomography will be referred to as OCT (Optical Coherence Tomography) hereinafter.
Capturing a wide-area fundus image by using a tomography apparatus such as those used in OCT allows three-dimensional observation of the state of the interior of the retinal layer. This makes it possible to quantify the state of a disease with an objective measure. Therefore, a tomography apparatus, as an apparatus for more accurate diagnosis of diseases, is increasingly expected to be used for wide-area fundus images.
Humans are involuntarily and incessantly making small eye movements (small involuntary eye movements) even when they are keeping close watch on a fixed point. For this reason, in the case of an apparatus such as those used with OCT which take a long time between the start of imaging and the end of imaging, an offset, distortion, and the like may occur between a plurality of captured tomograms due to the influence of the small involuntary eye movement of the eyes during imaging.
FIG. 24 is a view showing how an offset and distortion occur between tomograms due to the small involuntary eye movement of the eyes during imaging. Lines 2420 indicated by 24a of FIG. 24 represent imaging positions on a portion of an eye subjected to tomography. An OCT apparatus scans the highest line of the plurality of lines constituting the lines 2420 from the upper left on the drawing surface to the upper right on the drawing surface first. When the scanning position reaches the right end on the drawing surface, the OCT apparatus moves the imaging position to the next highest line, and scans it in the same manner as described above. Repeating such scanning up to the lowest line will capture a tomogram.
Lines 2430 indicated by 24b of FIG. 24 represents actual imaging positions when the OCT apparatus scans in accordance with the lines 2420 indicated by 24a of FIG. 24. As indicated by 24b of FIG. 24, the actual imaging positions are offset because of nonuniform scanning positions as well as the irregular intervals between the scanning lines due to the influence of the small involuntary eye movement of the eye portion during imaging.
To solve this problem, there has been a proposal for correcting the positional offsets between tomograms accompanying imaging position offsets by removing the influence of small involuntary eye movement during imaging. For example, Japanese Patent Laid-Open No. 2007-130403 has proposed an arrangement for correcting the positional offsets between two or more tomograms by using a reference image (one tomogram perpendicular to two or more tomograms or a fundus image).
As disclosed in patent reference 1 described above, however, in order to calculate a direction perpendicular to two or more tomograms and obtain a reference image used in the process of returning the scanning position to the origin, it is necessary to extract, in advance, feature amounts effective for calculation from the respective tomograms. In the case of a portion of an eye, the retina has a layered structure, and hence is lacking in feature amounts effective for the calculation of a direction perpendicular to each tomogram. It is therefore considered difficult to accurately correct positional offsets between tomograms by using the method disclosed in patent reference 1.